The invention concerns a burner for producing a flow of hot gas, in particular to shrink a plastic foil, with a pressure-distributing chamber of a fan-shaped contour connected to mixture feed-means and merging at its wide end into a combustion chamber, a flame-holder being mounted in the transition zone between the pressure-distribution and combustion chambers.
Such burners are needed to shrink plastics, for instance in the form of wraps which are drawn on a stack of goods located on a pallet. The shrinkage can be carried out using manual implements, but also with automatic equipment which is stationary. It is important that a uniform, flame-free flow of hot gas be produced when shrinking. Especially where manual implements are used, the widest possible exhaust of the burner exhaust gases is desired. Accordingly the combustion chamber and its discharge are designed with an elongated cross-section.
A burner of the initially cited species is known (German patent application No. P 32 15 608.0). The flame-holder in this burner is formed by a plurality of bars parallel to one another and extending above the height of the rectangular cross-section at the beginning of the combustion chamber. Because such a flame-holder constitutes a substantial flow impedance, it also effects simultaneously a distribution of the gases flowing from the pressure distributor into the combustion chamber. The assumption in this case is that the individual bars of the flame-holder are positioned at relatively precise spacings from each other. However such an accurate arrangement can be achieved only with difficulties in actual manufacture. Changes in the pressure distribution in the combustion chamber and hence irregularities in the outflowing hot gas however are incurred already for rather small deviations.
The object of the invention is to so improve a burner of the initially described species that it evinces reliable and uniform properties with simple design.
This problem is solved in that the flame holder is a bar of shaped section which extends over the entire width of the pressure distribution chamber and combustion chamber and which together with the adjoining walls forms passageway-gaps for the combustion gases.
In this burner the flame holder is reduced to a single component which even in manufacture can be very precisely positioned using simple means. Even when making large numbers, reliable and constant properties of the burners can be realized. The bar-shaped flame holder equally divides the flow of gas entering the combustion chamber. The partial flows entering the combustion chamber through the passageway-gaps are drawn by the effect of the dead water formed behind the shaped-section bar into the center of the combustion chamber and therefore less stress the combustion chamber walls when being burned.
Particularly advantageous conditions arise when the flame holder is a tube producing to the side of the flow a pronounced dead-water area which not only causes the desired guidance of the partial flows toward the center of the combustion chamber, but also generates intense turbulence of the combustion gases. This is especially the case when the passageway-gaps are of the same dimensions.
A preferred embodiment of the invention is characterized in that the essentially plane walls of the pressure-distribution and combustion chambers join in a kink in the transition zone and in that the tube is mounted symmetrically to the kink. In such a case passageway-gaps with Venturi-like cross-sections are produced between the outsides of the tube and the adjoining walls.
To make possible complete combustion of the combustion gases before they are discharged from the combustion chamber, the legth of the combustion chamber, seen in the direction of flow of the combustion gases, should be at least three times the height of the combustion chamber which is constant over its length.
To improve the pressure distribution across the width of the combustion chamber, the pressure distribution chamber may be additionally provided with installations to guide and distribute the combustion gases. These installations in particular should be so designed and arranged that always the same pressure conditions prevail across the width of the combustion chamber and that always the same rates of combustion gases enter the combustion chamber.
The installations can be made in particular of sheetmetal or plate essentially extending across the pressure distribution chamber and forming together with the walls of the pressure distribution chamber a passageway-gap. In addition the sheetmetal or plate can also be perforated .